a) Field of the Invention
The invention is directed to a radiation source for generating extreme ultraviolet (EUV) radiation based on a hot, dense plasma generated by gas discharge, particularly for generating high average EUV radiation outputs.
b) Description of the Related Art
In the last 35 years, semiconductor chip producers have achieved considerable growth rates and increases in output by continuously reducing transistor sizes from the micrometer range to the nanometer range. Since its formulation in 1965, Moore""s law has been steadily corroborated in the semiconductor lithography industry by a gradual reduction of the wavelength in the utilized radiation. At present, the industry is making the transition from the ArF excimer laser with a wavelength of xcex=193 nm to the F2 laser with a wavelength of xcex=157 nm. There is a conviction that because of the transmission limits of lens systems radiation at xcex=157 nm will be the smallest radiation ever used in semiconductor lithography which utilizes transmission optics or catadioptric systems.
However, the increase in the operating speed of a microprocessor predicted for the end of this decade by Moore""s law could stagnate if the resolution limit of exposure equipment given by Rxcx9cxcex/NA for a resolvable structure spacing R is reached. This equation shows that the structure resolution can only be improved by reducing the wavelength xcex and/or increasing the numerical aperture NA of the optics. Since the theoretical limit of the numerical aperture NA is 1 and the industry already uses values up to NA=0.8, the sole possibility for reducing the resolution limit and, therefore, further reducing transistor size is a further reduction in wavelength.
Therefore, it can be stated at the present time that a further substantial increase in the numerical aperture of optics is impossible and that no transmission optics or catadioptric system permits the use of wavelengths substantially smaller than 157 nm. Accordingly, there was reason to fear that the development predicted by Moore""s law would stagnate in coming years if no alternative possibilities were found for overcoming the problem. Fortunately, the development of multilayer mirrors with a 70-% reflection factor in the range of 10 to 15 nm offered the semiconductor industry a new prospect for the use of EUV radiation in this wavelength range and accordingly provided new hope that current lithographic chip fabrication will remain for another decade as dynamic as it has been thus far.
Although radiation sources based on plasma generated by gas discharge as well as laser-generated plasmas have shown adequate potential to emit EUV radiation in the desired wavelength range of 10 to 15 nm, these sources are still far from being used as commercial high-output radiation sources such as are required in chip fabrication for exposure machines with output powers of several hundred watts. With the greatest possible conversion efficiency that can be achieved for a plasma generated by gas discharge estimated at about 1%/2xcfx80xc2x7sr, an input power of 20 kW would be required to collect 100-watt EUV radiation in a solid angle of xcfx80sr. Further, it must be kept in mind that the majority of this enormous power for converting into plasma must be transmitted over discharge surfaces of a few square centimeters. It can easily be imagined that these small surfaces will not be stable over a long duration, so that radiation sources based on a gas discharge appear unsuitable for stable long-term use due to the fact that they must work in continuous operation for upwards of at least twenty hours and more at repetition frequencies of between 2 and 10 kHz for commercial use in chip lithography.
Therefore, it is the primary object of the invention to find a novel possibility for the realization of an EUV radiation source which achieves a high average radiation output in the EUV region and remains stable for a sufficiently long period of time.
According to the invention, in a radiation source for the generation of extreme ultraviolet (EUV) radiation based on a dense, hot plasma generated by gas discharge containing two electrodes which are electrically separated from one another by insulators which are resistant to breakdown and at the same time form rotationally symmetric electrode housings for parts of a vacuum chamber, wherein a gas discharge for plasma generation is provided between a first electrode housing and a second electrode housing within the vacuum chamber and an exit or outlet opening for the radiation emitted by the plasma is provided in the first electrode housing, further containing a gas supply unit for generating a flow of working gas through the vacuum chamber, a high-voltage module for providing high-voltage pulses at the electrodes and a preionization unit for generating preionization of the working gas prior to the gas discharge triggered by the high-voltage pulse, the above-stated object is met, according to the invention, in that the second electrode housing has a narrowed portion and an electrode collar which adjoins the latter and which is enclosed concentrically by the first electrode housing, wherein a concentric insulator layer is provided in this area of concentric overlapping between the first electrode housing and the electrode collar of the second electrode housing in order to shield the concentric surface regions of the two electrode housings, which concentric insulator layer extends in the direction of the outlet opening of the first electrode such that the gas discharge takes place substantially only parallel to the axis of symmetry of the electrode housing, and the electrode collar is stepped radially relative to the concentric insulator layer in such a way that at least one end region of the electrode collar is at a distance from the concentric insulator layer such that a concentric gap is formed.
The outlet opening in the first electrode housing advantageously has the shape of a circular narrowed portion coaxial to the axis of symmetry of the electrode housing and the first electrode housing is expanded conically following the narrowed outlet opening, so that the gas discharge is ignited between the two electrodes in the interior of the first electrode housing and the dense, hot plasma is formed within the conical expansion after the outlet opening of the first electrode housing.
For purposes of suitable orientation of the gas discharge in the interior of the first electrode housing, the electrode collar of the second electrode housing projecting into the first electrode housing preferably has the shape of a hollow cylinder with a plurality of steps.
In this connection, it can be advantageous that the electrode collar is a hollow cylinder with two outer and one inner step, wherein the second outer step forms a transition from the electrode collar to the base body of the second electrode housing. Further, it is useful when at least one of the steps of the hollow cylinder has a conical transition in order to improve heat dissipation and the stability of the electrode collar relative to the base body of the second electrode housing.
The base body of the electrode housing is advantageously produced from one of the metals, copper, tungsten, molybdenum or a tungsten-copper alloy in a desired mixture ratio, wherein at least highly loaded zones of the electrode collar of the second electrode housing are produced from an alloy of tungsten with one of the materials, titanium, tantalum, zirconium, rhenium, lanthanum, lanthanum oxide, nickel, iron, nickel-iron compounds or zirconium-oxygen compounds in a desired mixture ratio, or the highly loaded zones comprise an alloy of molybdenum with one of the materials, titanium, tantalum, zirconium, rhenium, lanthanum, lanthanum oxide, nickel, iron, nickel-iron compounds or zirconium-oxygen compounds in a desired mixture ratio.
Zones of the electrode housing upon which the radiation flow acts particularly intensively, particularly free inner edges of the electrode collar or of the outlet opening, are coated, in addition, with a material having a low sputter rate. Coatings with aluminum oxide, aluminum nitride, zirconium oxides or silicon oxides are particularly suitable for this purpose.
Another advisable possibility for reducing electrode wear consists in coating highly loaded zones of the electrode housing, particularly the electrode collar or the outlet opening, with an alloy of tungsten, molybdenum or rhenium with one of the compounds aluminum nitride, aluminum oxide, zirconium oxide or silicon oxide. Further, coating these highly loaded electrode zones with a tungsten-carbon compound, preferably a tungsten-diamond compound, has proven particularly suitable.
It is advisable for the operation of the radiation source that the first electrode housing is arranged as anode and the second electrode housing is arranged as cathode for the high-voltage gas discharge. In another preferred variant, the first electrode housing is arranged as cathode and the second electrode housing is arranged as anode.
In order to prolong the life of the electrodes, it is further advisable when the first electrode housing and the second electrode housing are fashioned in such a way that they have a base body comprising material with very good thermal conduction, particularly copper, wherein an efficient heat dissipation system is joined to this base body for efficient elimination of heat from the discharge zone of the electrodes.
The heat dissipation system is preferably based upon a porous metal structure through which coolant is pumped under high pressure or upon a heat pipe system. In either case, water, a low-viscosity oil, e.g., Galden, mercury, sodium or lithium, can be used as active coolant.
It proves advantageous when a heat dissipation system of the type mentioned above is integrated in the base body of each electrode housing. However, it can also be arranged externally so that it is possible to exchange the electrode housings and heat dissipation system separately.
The concentric insulator in the interior of the first electrode housing which is provided for shielding the side walls of the first electrode housing from the electrode collar of the second electrode housing is advisably produced as an insulator pipe from one of the compounds, Si3N4, Al2O3, AlN, AlZr, AlTi, BeO or lead-zirconium-titanate (PZT).
The preionization module is advantageously arranged coaxially inside the second electrode housing and comprises two circular electrodes with a rod-shaped insulator located therebetween, wherein an end surface of the second electrode housing is advisably used as one of the circular electrodes and the surface of the rod-shaped insulator is provided for a sliding discharge for preionization of the working gas. In this connection, the rod-shaped insulator is preferably made of one of the materials, Si3N4, Al2O3, AlN, AlZr, AlTi, BeO, or of highly dielectric materials such as lead-zirconium-titanate (PZT), barium titanate, strontium titanate, lead borosilicate or lead-zinc borosilicate.
At the same time, the preionization module can have a gas inlet for the working gas, this gas inlet being guided coaxially through the rod-shaped insulator. Another advantageous way to supply the working gas consists in that a gas inlet with inlet openings that are evenly distributed with respect to the axis of symmetry is arranged in the conical expansion of the first electrode housing.
One of the gases, xenon, krypton, argon, neon, nitrogen, oxygen or lithium, or a mixture of some of the latter can be used as working gas. Xenon in a desired mixture ratio with one of the gases, hydrogen, deuterium, helium or neon, has proven to be a particularly suitable working gas.
In order to achieve sufficiently high average output power of the radiation source, the high-voltage module advisably contains a pulse generator with a repetition frequency between 1 Hz and 20 kHz for igniting the gas discharge and generating a dense, hot plasma.
In a radiation source for generating extreme ultraviolet (EUV) radiation based on a dense, hot plasma generated by gas discharge, preferably using hollow cathode triggered pinch arrangements, theta pinch arrangements, plasma focus arrangements or astron arrangements, containing two electrodes which are electrically separated and which at the same time form rotationally symmetric electrode housings for parts of a vacuum chamber, wherein a gas discharge for plasma generation is provided between the electrode housings inside the vacuum chamber, and an outlet opening for the radiation emitted by the plasma is provided in at least a first electrode housing, a gas supply unit for generating a flow of working gas through the vacuum chamber, a high-voltage module for providing high-voltage pulses to the electrodes, the above-stated object is further met, according to the invention, in that a second electrode housing likewise has a narrowed portion which is coaxially received by the first electrode housing, and each of the electrode housings comprises a base body with very good heat conduction which is connected to an efficient heat dissipation system and electrode zones subject to high thermal loading comprise materials with a high melting point at least at the narrowed portions of the electrode housings.
The first electrode housing is advantageously coated with an insulator layer at the inner surfaces coaxially adjoining (in an electrically insulated manner) the narrowed portion of the second electrode housing, so that the gas discharge is oriented essentially only parallel to the axis of symmetry of the electrode housings.
Further, it has proven particularly advisable when the outlet opening of the first electrode housing is a circular narrowed portion coaxial to the axis of symmetry of the electrode housing and the electrode housing is expanded conically after the outlet opening, so that the gas discharge between the two electrodes is ignited and the dense, hot plasma is formed inside the conical expansion after the outlet opening of the first electrode housing.
The highly loaded electrode zones preferably comprise tungsten or molybdenum or an alloy of tungsten or molybdenum with one of the materials, titanium, tantalum, zirconium, rhenium, lanthanum, lanthanum oxide, nickel, iron, nickel-iron compounds or zirconium-oxygen compounds in a desired mixture ratio.
In order to protect especially highly loaded parts of the electrode housings that are exposed to the radiation flow emitted from the plasma, the inner edges of the electrodes in particular are advantageously coated with materials having low sputter rates such as aluminum oxide, aluminum nitride, zirconium oxides, silicon oxides or an alloy of one of these compounds with tungsten, molybdenum or rhenium. Another possibility for protecting against erosion of parts of the electrode housing that are especially loaded by radiation consists in that the inner edges of the electrodes are coated with tungsten-carbon compounds, particularly with a tungsten-diamond compound.
The heat dissipation system connected to the electrode housings preferably contains a porous metal structure or heat pipe system in the base body.
In an electrode configuration in which at least a substantial portion of an electrode lies within an external electrode housing, the heat dissipation system has cooling channels for the inner electrode, wherein the cooling channels through the outer electrode housing are provided for cooling the inner electrode based on a porous metal structure or a heat pipe system.
The basic idea of the invention is founded on the consideration that present EUV radiation sources based on a gas discharge plasma can not meet the exacting requirements of lithography exposure devices for the semiconductor industry above all because enormous electrode wear apparently makes long term use impossible. On the one hand, the electrodes are exposed to considerable thermal loading and, further, are subject to an embrittlement effect through the intense radiation from the generated plasma which contains not only the desired EUV light, but also hard x-ray radiation and matter in the form of neutral particles and charged particles. On the other hand, the shape of the vacuum chamber and the electrode configuration located therein cause additional effects which lead to malfunctions even after brief use in continuous operation due to metallization of insulator surfaces. According to the invention, these unwanted effects are countered in that the active electrode zones are designed in such a way that a directed gas discharge is ignited in a defined manner and metallization of the insulator surfaces is extensively prevented. By means of further suitable shaping of an electrode housing, the location of the generated dense plasma is relocated from the actual gas discharge area to behind the termination of the discharge zone of the vacuum chamber provided as conventional outlet opening. Additional measures involve the choice of material of the base body of the electrodes and the highly loaded electrode zones and a coating of the inner surfaces of the electrodes for reducing sputter of electrode surfaces (common cathode sputter as well as sputter due to radiation-induced surface embrittlement). Another focal point for reducing electrode wear is the arrangements for effective cooling of the electrodes by means of porous metal structures or heat pipe systems (e.g., with porous tungsten-lithium heating pipes) in order to draw off heat loading of multiple kW/cm2.
With the radiation source according to the invention it is possible to achieve a stable plasma generation for emission of EUV radiation through reduction of electrode wear and other effects (e.g., metallization of insulator surfaces) impairing the discharge behavior in the vacuum chamber, a high average radiation output in the EUV range, and long-term stability of sufficient extent.
The invention will be described more fully in the following with reference to embodiment examples.